I don't agree with advice saying to eat some superfree food after your meal, as this is just encouraging you to eat more food when you might not necessarily be hungry.
If you know your meal isn't going to have much superfree foods in it, then you should have the superfree food BEFORE your meal, perhaps as a salad or bowl of soup, then have your meal, you may find you do not need as much food as you would of had if you had served up that plate without the superfree because you are now not as hungry.
The reason slimming world advise to eat a 1/3 superfree with each meal is so that you don't overindulge in too much carbs or proteins. While still free foods, they are the higher in calories food, so eventually if you do eat too many of them, with not enough of the lower calorie superfree food, you will be consuming far more calories than your body physically needs.
Slimming world of course is not a calorie counting diet, but it's advice is very clever in that it is still controlling the calories you consume, but without having to count them every day. Hope that makes sense.